Scientific Forecaster Discussion

Synopsis... a weak low pressure will lift northeastward along a surface trough through Monday night as a stationary front remains well inland. High pressure will build in Tuesday and Wednesday before an inland trough of low pressure develops Thursday. A cold front will drop into the area next weekend.

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Near term /until 6 am Monday morning/... for the early evening update...a nose of strong low-level convergence is helping to maintain convection over the far southern zones early this evening. Have adjusted near term probability of precipitation to increase values to the 70 percent range from Ludowici to Savannah metropolitan to account for this. Already seeing impressive surface moisture flux convergence in excess of 45 g/kg/12hr over the coastal waters. This will continue to intensify this evening and overnight.

02/22z surface...radar and satellite analysis placed a stationary front from eastern Alabama through central Georgia...the central midlands into coastal plain of North Carolina. Weak low pressure /1009 mb/ was located right along the extreme eastern coast of Apalachee Bay south of Adams Beach flordia. The low was attached to southwest-northeast oriented surface trough which extended from the eastern Gulf of Mexico into far southeast Georgia. The low is forecast to meander north/northeast along the trough tonight while remaining well detached from the inland stationary front.

The broad cyclonic flow induced by the surface low will support increasing nocturnal speed convergence along the Georgia and lower South Carolina coast overnight as nocturnal jetting takes hold along the western wall of the stream. This coupled with high atmospheric moisture characterized by a ribbon of precipitable waters 2.10-2.30 inches will support a gradual uptick in showers/thunderstorms over the coastal waters after midnight. Low-level trajectories favor this activity moving onshore along the southern South Carolina coast and possibly parts of the Georgia coast early Monday. Will show probability of precipitation ramping up east of the I-95 corridor midnight-sunrise with probability of precipitation ranging from 20 percent to 40-50 percent at the coast... highest in the Hilton Head-Summerville-Moncks Corner corridor. Lows will range from the Lower-Middle 70s inland with upper 70s/near 80 at the beaches/downtown Charleston.

There will be a continued risk for localized flooding of low lying and poor drainage areas.

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Short term /6 am Monday morning through Wednesday/... Monday and Monday night...significant changes were made to the forecast as models have trended much wetter through Monday night. The main cause of this trend is the weak area of low pressure currently near The Big Bend of Florida that is forecast to lift northeastward along a stalled front. As this occurs...a deep south to southeast moisture feed will become enhanced ahead of the surface low and into coastal southeast South Carolina and southeast Georgia. The percent of normal precipitable water satellite product currently shows values in the 120-130 percent range near the low...and this airmass will become solidly entrenched across the forecast area beginning Monday morning. Such percent of normal values point to precipitable waters on the order of 2.25 inches for much of the day on Monday and into Monday night. The models...especially the 02/12z European model (ecmwf)...put the quantitative precipitation forecast bullseye right on the forecast area and mainly along the South Carolina coast. This seems quite reasonable given the degree of moisture...presence of a low level feature...and even some upper level divergence in the entrance of a 250-300 mb jet. Probability of precipitation have been increased into the 60 percent range for mainly the tri- County region...and this may need to be expanded depending on how precipitation evolves overnight into Monday morning. Overnight...the low will begin to lift northeastward and the axis of deeper moisture and best coverage will shift up toward the NC/SC border. Have maintained at least chance probability of precipitation across the east until a more definitive trend can be identified. Temperatures for Monday will be tricky as areas along and east of I-95 will likely see little to no direct sunlight. This could keep highs down significantly...with still low 90s possible across interior southeast Georgia.

Tuesday through Wednesday...much drier deep layer air will work into the area and probability of precipitation are no more than 20 percent as high pressure builds in behind the departing surface wave. Warm temperatures will return...with highs reaching the middle to upper 90s away from the immediate coast.

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Long term /Wednesday night through Sunday/... a relatively wet pattern is anticipated Wednesday night through Sunday as a longwave trough takes shape in the upper levels. A pronounced Lee trough will develop late week...then a cold front will drop into the area over the weekend. Precipitable waters surge back above 2" late week with values approaching 2.3" Saturday and Saturday night. We show increasing probability of precipitation during much of the period...especially during daytime hours. Temperatures will be moderated by lower heights and greater cloud cover...thus highs will mainly reach no higher than the lower 90s.

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Aviation /22z Sunday through Friday/... kchs...VFR this evening with increasing chances for MVFR ceilings/visibilities after 08z as scattered showers and possibly thunderstorms redevelop over the area. Timing for impacts is a bit in question...but the trend for scattered coverage is there. Will introduce vcsh by 08z with tempo group for MVFR visibilities/ceilings in moderate rain 09-12z to trend. The window for impacts may need to be moved or expanded based on later guidance and radar trends. Scattered showers will likely remain around the terminal for much of the morning hours with activity diminishing some by afternoon. Additional mention of showers/thunderstorms may be needed after 15z with the 06z taf cycle.

Ksav...light shower activity could impact the terminal 00-02z as a band of mainly light rain skirts by to the south. Current indications are that much of the night will remain VFR. Shower activity is expected to develop and remain east of the terminal...although it could come close at times after 09z. Rap/NAM soundings are similar in showing MVFR ceilings 09-14z so will go ahead an include this. Scattered showers/thunderstorms will likely be around the terminal for much of the afternoon Monday. The introduction of showers/thunderstorms may be considered with the 06z taf cycle as timing for possible impacts become a bit more certain.

Marine... tonight...a weak area of low pressure near the Florida Big Bend area will move northeast into southeast Georgia. This will place the tightest pressure gradient/low-level jet mainly beyond 20 nm and this is where we have raised a Small Craft Advisory. Across the near shore waters southerly winds will surge but remain mainly 20 knots or less with seas building up to 4 feet.

Monday through Friday...a weak area of low pressure will move northeastward along the coast through Tuesday. High pressure will then return with a trough of low pressure inland...with another front reaching the area late in the week. This setup will result in a prevailing southwest flow across the waters through the week...with various periods of enhanced flow as the pressure gradient tightens. Winds will generally top out in the 15-20 knots range with some potential for gusts to around 25 knots at times...mainly over the outer Georgia waters. Seas will be elevated through much of the week...especially Monday through Tuesday night. A Small Craft Advisory will be in effect for the outer Georgia waters through midday Tuesday to account for 6 feet seas. Otherwise...seas will be 2-4 feet out to 20 nm...and 5 feet beyond.

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Tides/coastal flooding... current data suggest tides will remain below critical thresholds for Charleston Harbor with this evening/S high tide cycle.